This invention relates to a method of generating gate signals of a pulse width modulation (PWM) inverter adapted for variable speed drive of an AC motor and a PWM inverter system using the method.
Of methods for generation of gate signals of a PWM inverter, such a method as disclosed in JP-A-59-25592 and copending U.S. patent application Ser. No. 380,039 entitled "PWM Inverter Apparatus" is known wherein two voltage vectors and a zero vector are used to determine gate signals by which the voltage vectors are caused to move along a magnetic flux circle. In the known method, duration times of the two voltage vectors are reserved within a control period (sampling period) and the remaining time is allotted to a duration time of the zero vector. Thus, the switching frequency of the PWM inverter is determined in the known method depending on the number of combinations of the voltage vectors and zero vector within the sampling period. Specifically, the greater the number of combinations of the voltage vectors and zero vector within the sampling period, the greater the switching frequency of the PWM inverter.
In the prior art, however, when the angular speed of the magnetic flux changes, the duration time of any of the voltage vectors and zero vector is rendered zero within the sampling period and as a result, the switching frequency of the PWM inverter is decreased. This raises problems that distortion of a voltage waveform is increased to entail an increase in noise in the motor.